EFIA/YB-1 is a component of cardiac HF-1A binding activity and positively regulates transcription of the myosin light-chain 2v gene

CARP, a cardiac ankyrin repeat protein, is downstream in the Nkx2-5 homeobox gene pathway

To identify the molecular pathways that guide cardiac ventricular chamber specification, maturation and morphogenesis, we have sought to characterize factors that regulate the expression of the ventricular myosin light chain-2 gene, one of the earliest markers of ventricular regionalization during mammalian cardiogenesis. Previously, our laboratory identified a 28 bp HF-la/MEF-2 element in the MLC-2v promoter region, which confers cardiac ventricular chamber-specific gene expression during murine cardiogenesis, and showed that the ubiquitous transcription factor YB-1 binds to the HF-la site in conjunction with a co-factor. In a search for interacting co-factors, a nuclear ankyrin-like repeat protein CARP (cardiac ankyrin repeat protein) was isolated from a rat neonatal heart cDNA library by yeast two-hybrid screening, using YB-1 as the bait. Co-immunoprecipitation and GST-CARP pulldown studies reveal that CARP forms a physical complex with YB-1 in cardiac myocytes and immunostaining shows that endogenous CARP is localized in the cardiac myocyte nucleus. Co-transfection assays indicate that CARP can negatively regulate an HF-1-TK minimal promoter in an HF-1 sequence-dependent manner in cardiac myocytes, and CARP displays a transcriptional inhibitory activity when fused to a GAL4 DNA-binding domain in both cardiac and noncardiac cell context. Northern analysis revealed that carp mRNA is highly enriched in the adult heart, with only trace levels in skeletal muscle. During murine embryogenesis, endogenous carp expression was first clearly detected as early as E8.5 specifically in heart and is regulated temporally and spatially in the myocardium. Nkx2-5, the murine homologue of Drosophila gene tinman was previously shown to be required for heart tube looping morphogenesis and ventricular chamber-specific myosin light chain-2 expression during mammalian heart development. In Nkx2-5(−/−)embryos, carp expression was found to be significantly and selectively reduced as assessed by both whole-mount in situ hybridizations and RNase protection assays, suggesting that carp is downstream of the homeobox gene Nkx2-5 in the cardiac regulatory network. Co-transfection assays using a dominant negative mutant Nkx2-5 construct with CARP promoter-luciferase reporter constructs in cardiac myocytes confirms that Nkx2-5 either directly or indirectly regulates carp at the transcriptional level. Finally, a carp promoter-lacZ transgene, which displays cardiac-specific expression in wild-type and Nkx2-5(+/−) background, was also significantly reduced in Nkx2-5(−/−) embryos, indicating that Nkx2-5 either directly or indirectly regulates carp promoter activity during in vivo cardiogenesis as well as in cultured cardiac myocytes. Thus, CARP is a YB-1 associated factor and represents the first identified cardiac-restricted downstream regulatory gene in the homeobox gene Nkx2-5 pathway and may serve as a negative regulator of HF-1-dependent pathways for ventricular muscle gene expression.

Suppression of grp78 core promoter element-mediated stress induction by the dbpA and dbpB (YB-1) cold shock domain proteins

The highly conserved grp78 core promoter element plays an important role in the induction of grp78 under diverse stress signals. Previous studies have established a functional region in the 3' half of the core (stress-inducible change region [SICR]) which exhibits stress-inducible changes in stressed nuclei. The human transcription factor YY1 is shown to bind the SICR and transactivate the core element under stress conditions. Here we report that expression library screening with the core element has identified two new core binding proteins, YB-1 and dbpA. Both proteins belong to the Y-box family of proteins characterized by an evolutionarily conserved DNA binding motif, the cold shock domain (CSD). In contrast to YY1, which binds only double-stranded SICR, the Y-box/CSD proteins much prefer the lower strand of the SICR. The Y-box proteins can repress the inducibility of the grp78 core element mediated by treatment of cells with A23187, thapsigargin, and tunicamycin. In gel shift assays, YY1 binding to the core element is inhibited by either YB-1 or dbpA. A yeast interaction trap screen using LexA-YY1 as a bait and a HeLa cell cDNA-acid patch fusion library identified YB-1 as a YY1-interacting protein. In cotransfection experiments, the Y-box proteins antagonize the YY1-mediated enhancement of transcription directed by the grp78 core in stressed cells. Thus, the CSD proteins may be part of the stress signal transduction mechanism in the mammalian system.

Catecholamines and cardiac growth

The present knowledge concerning the alpha- and beta-adrenergic systems in the regulation of cardiac growth and gene expression is reviewed. To investigate the mechanism by which cAMP regulates the expression of cardiac genes we have used cultured myocytes derived from fetal rat hearts. We have shown previously that the addition of Br cAMP to the culture medium produced an increase in alpha-myosin heavy chain (alpha-MHC) mRNA level, in its rate of transcription as well as in the amount of V1 isomyosin. To characterize the promoter element(s) involved in cAMP responsive regulation of alpha-MHC expression we performed transient transfection analysis with a series of alpha-MHC gene promoter-CAT constructs. We have identified a 13 bp E-box/M-CAT hybrid motif (EM element) which conferred a basal muscle specific and cAMP inducible expression of the alpha-MHC gene. Using mobility shift assay we have documented that one of the EM element binding protein is TEF-1. Moreover, by incubating cardiac nuclear extracts with the catalytic subunit of PK-A we have found that factor(s) binding to the EM element is a substrate for cAMP dependent phosphorylation.

Combinatorial control of muscle development by basic helix-loop-helix and MADS-box transcription factors

Members of the MyoD family of muscle-specific basic helix-loop-helix (bHLH) proteins function within a genetic pathway to control skeletal muscle development. Mutational analyses of these factors suggested that their DNA binding domains mediated interaction with a coregulator required for activation of muscle-specific transcription. Members of the myocyte enhancer binding factor 2 (MEF2) family of MADS-box proteins are expressed at high levels in muscle and neural cells and at lower levels in several other cell types. MEF2 factors are unable to activate muscle gene expression alone, but they potentiate the transcriptional activity of myogenic bHLH proteins. This potentiation appears to be mediated by direct interactions between the DNA binding domains of these different types of transcription factors. Biochemical and genetic evidence suggests that MEF2 factors are the coregulators for myogenic bHLH proteins. The presence of MEF2 and cell-specific bHLH proteins in other cell types raises the possibility that these proteins may also cooperate to regulate other programs of cell-specific gene expression. We present a model to account for such cooperative interactions.

Atrial chamber-specific expression of the slow myosin heavy chain 3 gene in the embryonic heart

The quail slow myosin heavy chain 3 (slow MyHC 3) gene is expressed in the developing heart and in slow muscles of the developing limb. It is first expressed in the pulsatile cardiac tube in the embryo, and as the heart chamberizes its expression becomes restricted to the atria. To identify regulatory elements responsible for atrial-specific expression, the 5' upstream region of slow MyHC 3 gene was investigated. An atrial regulatory domain (ARD1) between -840 and -680 acts as an atrial cell-specific enhancer in primary cardiocyte cultures. ARD1 also specifies atrial-specific expression in vivo when the ARD1/heterologous promoter was introduced into developing chick embryos by a replication-competent retroviral vector. ARD1 is the first atrial cell-specific enhancer to be identified. Fine deletion and mutation analysis within ARD1 defined a 40-base pair vitamin D3 receptor-like element that controls atrial cell-specific expression of the slow MyHC 3 gene by inhibiting its expression in ventricular cardiocytes.

Transcriptional control of a nuclear gene encoding a mitochondrial fatty acid oxidation enzyme in transgenic mice: role for nuclear receptors in cardiac and brown adipose expression

Expression of the gene encoding medium-chain acyl coenzyme A dehydrogenase (MCAD), a nuclearly encoded mitochondrial fatty acid beta-oxidation enzyme, is regulated in parallel with fatty acid oxidation rates among tissues and during development. We have shown previously that the human MCAD gene promoter contains a pleiotropic element (nuclear receptor response element [NRRE-1]) that confers transcriptional activation or repression by members of the nuclear receptor superfamily. Mice transgenic for human MCAD gene promoter fragments fused to a chloramphenicol acetyltransferase gene reporter were produced and characterized to evaluate the role of NRRE-1 and other promoter elements in the transcriptional control of the MCAD gene in vivo. Expression of the full-length MCAD promoter-chloramphenicol acetyltransferase transgene (MCADCAT.371) paralleled the known tissue-specific differences in mitochondrial beta-oxidation rates and MCAD expression. MCADCAT.371 transcripts were abundant in heart tissue and brown adipose tissue, tissues with high-level MCAD expression. During perinatal cardiac developmental stages, expression of the MCADCAT.371 transgene paralleled mouse MCAD mRNA levels. In contrast, expression of a mutant MCADCAT transgene, which lacked NRRE-1 (MCADCATdeltaNRRE-1), was not enriched in heart or brown adipose tissue and did not exhibit appropriate postnatal induction in the developing heart. Transient-transfection studies with MCAD promoter-luciferase constructs containing normal or mutant NRRE-1 sequences demonstrated that the nuclear receptor binding sequences within NRRE-1 are necessary for high-level transcriptional activity in primary rat cardiocytes. Electrophoretic mobility shift assays demonstrated that NRRE-1 was bound by several cardiac and brown adipose nuclear proteins and that these interactions required the NRRE-1 receptor binding hexamer sequences. Antibody supershift studies identified the orphan nuclear receptor COUP-TF as one of the endogenous cardiac proteins which bound NRRE-1. These results dictate an important role for nuclear receptors in the transcriptional control of a nuclear gene encoding a mitochondrial fatty acid oxidation enzyme and identify a gene regulatory pathway involved in cardiac energy metabolism.

Factors involved in cardiogenesis and the regulation of cardiac-specific gene expression

The delineation of the mechanisms that regulate cardiac gene expression is central to our understanding of cardiac growth and development. Much progress has been made toward the identification of factors involved in tissue-restricted gene expression, especially in skeletal muscle cells. However, the mechanisms regulating the expression of cardiac-specific genes remain less well understood. Certain homeodomain proteins have been implicated in commitment to the cardiac phenotype. Among the best characterized are the murine proteins Csx, Nkx-2.5, and Nkx-2.6, related to the protein tinman, which is essential for heart formation in Drosophila. The expression of these genes precedes that of cardiac-specific genes and is therefore believed to play a critical role in the development of the heart. The GATA proteins are a family of zinc finger proteins that are also expressed early in cardiac development and may act separately from, or in concert with, the homeodomain proteins as crucial regulators of heart development. The myosin heavy and light chain genes, the actin genes, the troponin genes, and the atrial natriuretic factor and muscle creatine kinase genes have served as excellent paradigms for the study of cardiac gene expression. Although differences in cis-acting elements and their behavior in binding assays have been observed between different genes, there exist similarities that are noteworthy. In this review, we will discuss the factors involved in the regulation of cardiac-specific gene expression in an attempt to provide a better understanding of the process of cardiogenesis.

An HF-1a/HF-1b/MEF-2 combinatorial element confers cardiac ventricular specificity and established an anterior-posterior gradient of expression

The molecular determinants that direct gene expression to the ventricles of the heart are for the most part unknown. Additionally, little data is available on how the anterior/posterior axis of the heart tube is determined and whether the left and right atrial and ventricular chambers are assigned as part of this process. Utilizing myosin light chain-2 ventricular promoter/beta-galactosidase reporter transgenes, we have determined the minimal cis-acting sequences required for ventricular-specific gene expression. In multiple independent transgenic mouse lines, we found that both a 250 base pair myosin light chain-2 ventricular promoter fragment, as well as a dimerized 28 bp sub-element (HF-1) containing binding sites for HF1a and HF1b/MEF2 factors, directed ventricular-specific reporter expression from as early as the endogenous gene, at day 7.5-8.0 post coitum. While the endogenous gene is expressed uniformly throughout both ventricles, the transgenes were expressed in a right ventricular/conotruncal dominant fashion, suggesting that they contain only a subset of the elements which respond to positional information in the developing heart tube. Expression of the transgene was cell autonomous and its temporospatial characteristics not affected by mouse strain/methylation state of the genome. To determine whether ventricular-specific expression of the transgene was dependent upon regulatory genes required for correct ventricular differentiation, the 250 base pair transgene was bred into both retinoid X receptoralpha and Nkx2-5 null backgrounds. The transgene was expressed in both mutant backgrounds, despite the absence of endogenous myosin light chain-2 ventricular transcript in Nkx2-5 null embryos. Ventricular specification, as judged by transgene expression, appeared to occur normally in both mutants. Thus, the HF-1 element, directs chamber-specific transcription of a transgene reporter independently of retinoid X receptoralpha and Nkx2-5, and defines a minimal combinatorial pathway for ventricular chamber gene expression. The patterned expression of this transgene may provide a model system in which to investigate the cues that dictate anterior-posterior (right ventricle/left ventricle) gradients during mammalian heart development.

Transcription factors and the cardiac gene programme

During the past decade, major advances have been made in uncovering the mechanisms that switch genes on and off. Gene methylation and histones play an important role in gene (in)activation. Following gene activation, the initiation of transcription by RNA polymerase requires the assembly of multiple protein complexes on the promoter region of a gene. How a cell type-specific gene expression pattern can be induced is a key question in cardiovascular biology today. Members of the helix-loop-helix-family of the transcription factors play a dominant role in skeletal muscle formation. In cardiac muscle the situation is less obvious. Recent studies identified muscle transcription factors like MEF-2, TEF-1 and MNF, which are common to both the skeletal and cardiac muscle lineages. A few transcription factors, among which Nkx 2.5 and GATA-4, are expressed predominantly in the heart. The absence of master regulators in the heart points to the importance of interaction between ubiquitous factors and tissue restricted factors to initiate the cardiac gene programme and to lock these cells in their differentiated state. The recent development of murine transgenic and gene-targeting technology provides tools to study the role of mammalian transcription factors in vivo. Interesting cardiac phenotypes are found in gene targeted mice, indicating a crucial role for retinoic acid and homeobox genes in murine cardiogenesis.

Transcription and masking of mRNA in germ cells: involvement of Y-box proteins

Gametogenesis is directed by various specialized genetic mechanisms which, to a considerable extent, apply to the production of both eggs and sperm and have been conserved across a wide spectrum of eukaryotic organisms. Two key aspects which are discussed here are: germ-cell-specific gene transcription; and translational repression (masking) of mRNA accumulated in oocytes and spermatocytes/spermatids. Together, these two processes conspire to deliver often large amounts of essential proteins at the appropriate stages of development. It is perhaps not surprising that recent evidence points to a functional link between transcription activation and translation repression, both processes being determined in the nucleus and involving common components. One set of components which has been studied recently are members of the Y-box family of regulatory proteins. Most information of the involvement of Y-box proteins in germ cell development comes from studies on amphibian oocytes and mammalian spermatids. In these cells, Y-box proteins have been detected as major components of both maternal and paternal mRNP particles and have been shown to be instrumental in the masking process. Y-box proteins are also implicated in the regulation of several germ-cell-specific genes. Possible connections between these processes are discussed.

Inhibition of a signaling pathway in cardiac muscle cells by active mitogen-activated protein kinase kinase

Signaling via the Ras pathway involves sequential activation of Ras, Raf-1, mitogen-activated protein kinase kinase (MKK), and the extracellular signal-regulated (ERK) group of mitogen-activated protein (MAP) kinases. Expression from the c-Fos, atrial natriuretic factor (ANF), and myosin light chain-2 (MLC-2) promoters during phenylephrine-induced cardiac muscle cell hypertrophy requires activation of this pathway. Furthermore, constitutively active Ras or Raf-1 can mimic the action of phenylephrine in inducing expression from these promoters. In this study, we tested whether constitutively active MKK, the molecule immediately downstream of Raf, was sufficient to induce expression. Expression of constitutively active MKK induce ERK2 kinase activity and caused expression from the c-Fos promoter, but did not significantly activate expression of reporter genes under the control of either the ANF or MLC-2 promoters. Expression of CL100, a phosphatase that inactivates ERKs, prevented expression from all of the promoters. Taken together, these data suggest that ERK activation is required for expression from the Fos, ANF, and MLC-2 promoters but MKK and ERK activation is sufficient for expression only from the Fos promoter. Constitutively active MKK synergized with phenylephrine to increase expression from a c-Fos- or an AP1-driven reporter. However, active MKK inhibited phenylephrine- and Raf-1-induced expression from the ANF and MLC-2 promoters. A DNA sequence in the MLC-2 promoter that is a target for inhibition by active MKK, but not CL100, was mapped to a previously characterized DNA element (HF1) that is responsible for cardiac specificity. Thus, activation of cardiac gene expression during phenylephrine-induced hypertrophy requires ERK activation but constitutive activation by MKK can inhibit expression by targeting a DNA element that controls the cardiac specificity of gene expression.